If you are experiencing an issue trying to purchase SMS or World Credits from the Add-Ons screen, please try the following:
1) Sign out of Fongo Mobile app (Account > Logout)
2) Restart phone (hold Power and Home button until phone reboots)
3) Open Fongo App and login
4) Attempt to make purchase again.
If this still does not work Please try to re-install the Fongo App.